Mechanical forces acting on cells, or generated by cells, influence organ functions in many ways. Little is known, however, about the basic biophysical mechanisms that govern mechanics of the cell and, in particular, its ability to resist distortion of shape. The goal of this Project is to understand the relationship between the structure of the cell and its ability to resist distortion of shape. In this project, the central hypothesis Is that the resistance of the cell to shape distortion is provided by the cytoskeleton, organized as a discrete (as opposed to continuum), interconnected, prestressed structure. Specific predictions arising from this hypothesis will be tested, most notably that the cytoskeletal shear stiffness increases with increasing prestress and increases approximately linearly with increasing applied shear stress. Key molecular components and specific mechanism within the cytoskeleton that account for these mechanical properties of cells will be identified. The hypothesis predicts that microfilaments play a major role in providing prestress. These mechanisms will be modulated using pharmacological:means. The hypothesis predicts that bronchoconstrictors will increase cytoskeletal tension and, therefore, increase the prestress and as a result the shear stiffness. A theoretical framework will be developed to better understand the mechanical properties of the cell and their relationship to the underlying cellular microstructure. The key probe that will be utilized in this project is magnetic twisting cytometry. While the mechanism addressed here may apply to adherent cells of all kinds, this project will focus mostly, but not exclusively, on the human airway smooth muscle cell because of its relevance to many aspects of lung functions.